1. Field of the Invention
The present invention relates generally to an improvement in the present inventor's method or technique, as claimed in the related art, of conditioning a gas sensor through the application of pulse discharges in order to condition mixed-potential gas sensors for detecting gases commonly found in combustion exhaust.
2. Description of the Related Art
The sensing element used in zirconia oxygen sensors is generally formed of a zirconia thimble having an inner and outer metal coating, usually platinum, to form an electrode. The electrode is then used to measure the differential oxygen concentration between the measured gas on the outside of the thimble, and a reference gas, usually atmospheric, on the inside of the thimble. By measuring the voltage between two electrodes, the differential oxygen concentration can be calculated.
Solid electrolyte oxygen sensors comprising of gas impermeable zirconia ceramic separating two conductive (Pt) electrodes are widely used for combustion control in power plants as well as in the exhaust of automotive internal combustion engines. For utilization of oxygen sensors for industrial combustion control, the sensor must demonstrate certain performance criteria, i.e. a typical relative accuracy of between 3-5% (or absolute accuracy of 0.1-0.2%), a response time of less than 10 seconds, and a life expectancy typically greater than 1 year.
An improved method for activating such sensors is described in the inventor's own related prior art reflected in U.S. Pat. No. 7,585,402 and in the subsequent Continuation-in-Part of U.S. Ser. No. 11/152,971, filed Jun. 15, 2005 and still pending. A typical Schematic of the Pulse discharge technique is shown in FIG. 1 below, which is a representation of the sensor conditioning in accordance with the previous invention.
Sensors made with the inventor's own activation method result in numerous improvements in performance. Not only can such thimble sensors be used competitively with planar type designs as oxygen sensors, they can also be used as mixed potential sensors for the direct measurement of NOx and O2 consecutively with the same sensor electrode. While this dual function of a mix potential sensor can have those benefits described in the related art over planar type O2 sensors for current automotive applications (i.e. Exhaust Gas Oxygen (EGO)), it has been found that the mechanical stability of thimble type sensors and the stability of the sensor readings exceed performance of the planar sensors. Given that the stability of these planar sensors and their outputs are adversely affected by moisture, they can be used only in high dew point environments. While planar type sensors generally incorporate a heating element at the sensor surface so that the sensor surface temperature is known or controlled, the variation in the three dimensional geometry of thimble type sensors make such a solution unpractical.
Consequently, in order to obtain the benefits of stable sensor operation and high resolution at very low analyte concentrations it would be beneficial to be able to measure when the sensor impedance is within a proper range, and preferably to control the sensor impedance to within such a proper range.